We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of tidal disruption event (TDE) AT2018hyz, spanning 970–1300 d after optical discovery. In conjunction with earlier deeper limits, including those at≈ 700 days, our observations reveal rapidly rising emission at 0.8–240 GHz, steeper than F ν∝ t 5 relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (eg, off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multifrequency data allow us to directly determine the radius and energy of the radio-emitting outflow, and we find from our modeling that the outflow was launched≈ 750 days after optical discovery. The outflow velocity is mildly relativistic, with β≈ 0.25 and≈ 0.6 for a spherical geometry and a 10 jet geometry, respectively, and the minimum kinetic energy is E K≈ 5.8× 10 49 and≈ 6.3× 10 49 erg, respectively. This is the first definitive evidence for the production of a delayed mildly relativistic outflow in a TDE; a comparison to the recently published radio light curve of ASASSN-15oi suggests that the final rebrightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past nonrelativistic TDEs (eg, ASASSN-14li, AT2019dsg) and the relativistic TDE Sw J1644+ 57. We suggest that such delayed outflows may be common in TDEs.